Presume to become all-powerful (7)
I believe the answer is:
supreme
'all-powerful' is the definition.
(thesaurus)
'presume to become' is the wordplay.
'to become' indicates an anagram.
'presume' is an anagram of 'SUPREME'.
(Other definitions for supreme that I've seen before include "creamy dish of chicken perhaps" , "Most important" , "Cardinal, foremost" , "Ranked highest" , "Chief" .)